DEVELOPMENT OF THE THORACIC DUCT 79 



the strong impression that the observed variations in the relation 

 of the embryonic thoracic duct approach to the sympathetic 

 nerve hinge largely for their production upon the share which 

 the broncho-mediastinal duct is to take in individual cases in 

 the establishment of the preazygos segment of the thoracic duct 

 channel. (Compare relations of 35 and 37 in the schematic 

 cross sections shown above in the text figures.) 



If now this ventro-medial end piece of the thoracic duct 

 approach had effected its junction with the preazygos segment of 

 the thoracic duct along the line of the lymphatic anlages on the 

 medial side of the sympathetic and in the interval between this 

 strand and the vagus, then the individual condition already de- 

 scribed in series 142 (fig. 91) would have resulted. 



This same topographical relationship of sj^mpathetic nerve and 

 lymph channel is also seen in the 15.5 mm. embryo, series 143, 

 shown in reconstruction from the right side in Fig. 171. The 

 thoracic duct approach (12) intersects the sympathetic nerve (1) 

 on its ventro-medial aspect at an acute angle to establish its con- 

 nection with the cephalic end of the preazygos segment of the 

 thoracic duct (35). This type appears the prevalent one in the 

 embryos in my collection. 



I am strongly inclined, on the evidence of the embryos of my 

 collection, to regard the development of the thoracic duct approach 

 of the jugular lymph sac as occupying three distinct chronological 

 stages: 



1. Early stage. Embryos of 13-13.5 mm. 



Series 92 — 13. mm. 



Series 189—13.5 mm. 

 In this period the thoracic duct approach appears large, at 

 times multilocular, and shares the redundancy and capacious 

 lumen which characterizes the entire jugular lymph sac in the 

 earlier genetic period. 



2. Intermediate stage. Embryos of 14 mm. 



Series 210 — 14 mm. 

 Series 212 — 14 mm. 

 Series 122 — 14 min. 

 The sac, as a whole, and the thoracic duct approach in particu- 



